There is an increasing need for small, high efficiency heat engines for carbon-saving applications in the energy conservation and renewable energy industries. These applications include micro-combined heat and power, concentrated solar power and combined heat and power, heat pumps, power from waste and process heat, and power from small scale solid biomass systems. For many years the Stirling engine has been the favoured prime mover, largely because of the high theoretical efficiency of the Stirling cycle. However, in practice technical issues have prevented achievement of high fractions of that theoretical efficiency, and costs have remained high. As a result the Stirling engine has not been commercialised on a significant scale.
One reason for the lack of commercialisation of Stirling engines is that because the out-of-phase reciprocation of the Stirling engine's pistons creates reversing, unsteady charge gas flows with cyclically varying gas conditions, the heat exchangers that are crucial to the engine's performance can be specified for only one point on the cycle, and are non-optimal for the remainder of the cycle. It also results in anomalous heating and cooling of the charge gas after expansion and compression respectively. Both factors increase loads on the heat exchangers, and increase the need for air pre-heat at the hot end and for heat rejection at the cold end.
A second reason is that there is a severe heat transfer imbalance between the low pressure combustion gases and the high pressure charge gas, and a similar but less severe imbalance at the cooler. In the heater this is exacerbated by the normally cross-flow mode of heat transfer. Increasing the surface area density on the external side of the heater without significant increases in its internal volume, and providing counter-flow/parallel-flow heat exchange, would reduce the imbalance, but it is not possible to do this in a cost effective manner with conventional manufacturing methods.
These two factors contribute to the Stirling engine in general, and its heat exchangers in particular, being larger, more expensive and less efficient than theory alone would suggest.
Other engines and/or cycles have been proposed for these applications, and in some cases they have been used. Examples include rotary Brayton cycle micro-turbines, and organic Rankine cycles with an expander based on scroll compressor technology operating in reverse mode. The rotary and orbiting motions respectively of the components reduce, or in the case of the turbine eliminate, the unsteady flow problem of the Stirling engine. However, their theoretical cycle efficiencies are not as high as that of the Stirling engine, and, as with the Stirling engine, limitations on heat exchanger performance combine, with other factors, to prevent their meeting the requirements of their potential mass markets.
A partial solution to the heat exchanger issue is provided by WO-A-2006/064202 and WO-A-2008/047096, which disclose the design of ducts for heat exchangers manufactured by powder-based, additive layer, near netshape manufacturing techniques based on energy beams, such as laser beams. Such manufacturing techniques are known by various names: one such name is Selective Laser Melting (SLM). SLM can be used to manufacture compact heat exchanger/reactors as described in WO-A-2006/064202. Components made from SLM are normally built up on a thick metal platen, often with SLM-built supports between the component and the platen, and then removed from it by machining or other suitable methods after completion of the build. SLM allows the manufacture of more compact heat exchangers with higher surface/volume ratios, lower hydraulic diameters and almost complete freedom of 3-D design, in combinations which are otherwise not feasible with other manufacturing technologies.
Another potential solution is a Brayton cycle heat engine based on commercial scroll compressor technology. WO-A-2003/069130 discloses such a heat engine whose main mechanical components are a cold scroll compressor and a hot scroll expander, each of which normally comprises a casing, a stationary scroll and an orbiting scroll, with associated valves, ducts, bearings and other components. WO-A-2003/069130 also discloses a heating surface in the form of fins to an outside area of the casing of the scroll, and a heating chamber provided to an outer circumference of the scroll housing. The claimed advantage is that this provides a means of heating the charge gas while it expands in the expander and of cooling it while it is being compressed in the compressor, thus allowing the expansion and compression processes to approach closer to the ideal and most efficient isothermal gas processes possible. The advantage may be interpreted as a change in engine cycle from a Brayton cycle to the potentially more efficient Ericsson cycle. However, it is difficult for such fins to provide a means of increasing heat transfer sufficient to make a significant difference in efficiency.
WO-A-2006/008463 discloses a method of making solid structures and cylindrical components for a reciprocating piston engine in the form of a plurality of tubes spaced apart and held relative to each other by spacers formed by laser melting. The spacers may include fins to enhance heat transfer to fluid within the interstitial volumes between the tubes. The interstitial volumes may be evacuated, filled with fluid, filled with material that will solidify and exploited in a variety of different ways. Different interstitial volumes may be pressurised to different degrees to spread the stresses exerted upon the structure between the various thin-walled tubes. This permits mechanical and thermal loads to be spread through the structure formed of the plurality of tubes spaced apart and held relative to each other by the spacers formed by laser melting.